OBJECTIVE To assess the effect of reducing the rhesus monkey diet to contain levels of calcium and vitamin D similar to that consumed normally by humans. RESULTS Reduction of calcium intake to 0.15% does not increase markers of skeletal turnover. In contrast, reduction of vitamin D intake to levels comparable to humans leads to a marked reduction in serum vitamin D concentration (levels comparable with those of humans with osteomalacia.). KEY WORDS Osteoporosis, age-related, vitamin D, skeletal turnover FUNDING Geriatrics section discretionary funds